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be both simpler and substantially more efficient. Rather than use a hand-rolled iteration technique that isn't quite the same as RPO, use the pre-built RPO loop body traversal utility. Once visiting the loop body in RPO, we can assert that we visit defs before uses reliably. When this is the case, the only need to iterate is when simplifying a def that is used by a PHI node along a back-edge. With this patch, the first pass over the loop body is just a complete simplification of every instruction across the loop body. When we encounter a use of a simplified instruction that stems from a PHI node in the loop body that has already been visited (due to some cyclic CFG, potentially the loop itself, or a nested loop, or unstructured control flow), we recall that specific PHI node for the second iteration. Nothing else needs to be preserved from iteration to iteration. On the second and later iterations, only instructions known to have simplified inputs are considered, each time starting from a set of PHIs that had simplified inputs along the backedges. Dead instructions are collected along the way, but deleted in a batch at the end of each iteration making the iterations themselves substantially simpler. This uses a new batch API for recursively deleting dead instructions. This alsa changes the routine to visit subloops. Because simplification is fundamentally transitive, we may need to visit the entire loop body, including subloops, to handle knock-on simplification. I've added a basic test file that helps demonstrate that all of these changes work. It includes both straight-forward loops with simplifications as well as interesting PHI-structures, CFG-structures, and a nested loop case. Differential Revision: https://reviews.llvm.org/D47407 llvm-svn: 333461 |
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